The colony of Brazilian free-tailed bats (Tadarida brasiliensis) at Carlsbad Cavern, New Mexico, is a wellknown example of this highly gregarious and conspicuous species in North America. For nearly a century researchers have tried to estimate the size of this colony, but different census methods and lack of repeatability have resulted in questionable estimates that have given rise to poorly understood but highly popularized, longterm population trends for this migratory species. In this study we present accurate seasonal estimates of colony size based on a recently developed census method—thermal infrared imaging and computer vision analysis. The size of the colony was estimated several times monthly from March through October 2005. Our estimates range from 67,602 to 793,838 bats, values that are orders of magnitude lower than the largest historic estimates. Consecutive estimates of nightly emergences show fluctuations of as many as 291,000 individuals, indicating that colony composition is considerably more dynamic than previously thought. Our results, combined with a quantitative analysis of emergence behavior, question the validity of early historic estimates that millions of bats once roosted in this cave and suggest that the long-term pattern of decline reported for this species might not be as severe as currently thought. DOI: 10.1644/08-MAMM-A-391R.1.